Alumina ceramics which are excellent in insulating properties are used for a vacuum insulation vessel of a vacuum valve which has a pair of contact points which can be brought into contact or out of contact with each other
On the other hand, the latest vacuum valves are in a tendency of high-voltage, and improvement measures for electric strength in a vacuum, using electric field relief of electrodes or area effect exerted on a breakdown electric field are implemented.
In such improvement measures for the electric strength, although properties between vacuum gaps improve, there is a limit in improvement of properties in surface insulation of the vacuum insulation vessel.
That is, a surface dielectric breakdown in the vacuum somewhat differs from a dielectric breakdown between the vacuum gaps as a phenomenon. Once a field electron emitted from the electrodes is charged on the surface and arrives at a critical field, it emits a secondary electron and will result in the dielectric breakdown.
Although the control of electrification can be carried out by adding other ingredients to the vacuum insulation vessel and decreasing its resistivity, there is a limit in controlling the electrification without changing fundamental ingredients.
When the electrification occurs, it is detected as partial discharge with luminescence.
For these reasons, it is desired to improve surface insulating properties without changing the ingredients of the alumina ceramics.
Here, regarding the vacuum valve whose outer circumference is molded by an epoxy resin, since its external insulation is reinforced, at least an improvement of properties in the surface insulating which is internal insulation in a vacuum is desired.